1. Field of the Invention
The invention relates to image/video processing techniques.
Specifically, the invention relates to de-mosaicing artifact removal, and was developed by paying attention to the possible application to digital cameras. Reference to this possible field of application is not however to be construed in a limiting sense of the scope of the invention.
2. Description of the Related Art
Digital still cameras include two-dimensional image sensing arrays, such as a Charge Coupled Device (CCD) that integrates incident scene light over a predetermined time to provide an electronic information signal corresponding to the scene light intensity incident to the array. Such a two-dimensional image-sensing array comprises a predetermined number of discrete image sensing elements or pixels, which respond to incident illumination to provide an electronic information signal corresponding to the intensity of the incident illumination.
In order to record color images through a single sensor, the surface of the sensor is covered with a mosaic of colored filters. This kind of sensor is called a Color Filter Array (CFA). In essence, each pixel has its own spectrally selective filter to peer through. The most popular color filter array pattern used today is the Bayer Pattern, which is shown in FIG. 1.
Since each image sensing element can only detect one color of illumination (R=Red, G=Green, B=Blue), the color information for the other colors not detected by that image sensing element must be filled in; thus achieving a “de-mosaicing” effect, which is usually achieved by interpolation.
Conventional types of interpolation, however, can provide images with objectionable aliasing artifacts such as “color fringes” near sharp edges. There are two main types of de-mosaicing artifacts, named false colors and zipper effect. False colors are those artifacts corresponding to noticeable color errors as compared to the original, non-mosaiced image. The zipper effect refers to abrupt or unnatural changes of color differences between neighboring pixels, manifesting as an “on-off” pattern.
A conventional approach to solve these problems is to eliminate the color fringes at the expenses of image sharpness by blurring the picture, so that the edges are not sharp enough to create a color fringe. Blurring the image in this manner, however, has its obvious disadvantages resulting in a reduction in resolution. Therefore, it is necessary to provide a de-mosaicing artifact removal technique that reduces color fringing without the amount of blurring otherwise required.
A technique to resolve color fringes without blurring the images is proposed in U.S. Pat. No. 4,724,395.
The approach disclosed in U.S. Pat. No. 4,724,395 stems from the consideration that natural images exhibit a strong correlation of the red, green and blue channels, especially for high frequencies, so that they are likely to have the same texture and edge locations.
Because of this inter-channel correlation, the difference between two colors in a neighborhood is nearly constant, while it rapidly increases and decreases in the area of sharp gray edges, where color interpolation has introduced false colors. The approach of U.S. Pat. No. 4,724,395 applies a median filter to the inter-channel differences in a neighborhood and then uses the resulting value to force pixels with distinct colors to be more similar to their neighbors, avoiding false colors. Moreover, the Color Filter Array color value at each pixel is not altered.
In general, the approach of U.S. Pat. No. 4,724,395 is rather effective in suppressing de-mosaicing artifacts, while preserving sharp edges. However, some de-mosaicing artifacts still remain around sharp edges and fine details. This is partly due to the fact that each pixel has independent inter-channel differences, and filtering the differences separately does not take into account the spectral correlation between color planes.
The arrangement disclosed in Wenmiao Lu, Yap-Peng Tan, “Color Filter Array De-mosaicing: New Method and Performance Measures”, IEEE Transactions on Image Processing, vol. 12, no. 10, Oct. 2003, incorporates median filtering with the spectral correlation for more effective suppression of de-mosaicing artifacts by lifting the constraint of keeping the original CFA-sampled color values intact. Furthermore, Lu et al. make use of the latest processed color values to filter the subsequent pixels so that estimation errors can be effectively diffused into local neighborhoods.
To sum up, the approach of U.S. Pat. No. 4,724,395 does not remove all the artifacts introduced by conventional color interpolation techniques, and moreover, under certain conditions, produces an annoying zipper effect in the horizontal and vertical directions.
Conversely, the approach of Lu et al. removes more extensively false colors and artifacts in comparison with the approach of U.S. Pat. No. 4,724,395, but considerably blurs the images, because it adjusts the green channel of each pixel by means of an average of both the red and blue values of the same pixel.